# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/(?:\G(?!^)|^((?:\/[^\/\n]*){3}\/))([^\d\n]*)\d/$1$2#/gm;t' <<< "/path/to/something/1234/end
/path/to/something/12/1234/end
/FWS/teis-facade-rest_v712/flk/tariffs/114776/14/options
/mem-utms-facade/v1.2/rest/URA/ST00006486/note
/FWS/spg-facade/eis/2650114465322000006/contract
/FWS/fanotical-soyarty-service/flk/lk_2456882/bonusFlk"
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for SED, please visit: https://www.gnu.org/software/sed/manual/html_node/The-_0022s_0022-Command.html